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\section{Ngstat}
Formula for variance of watterson:
$\text{Var}(\theta_W)=\frac{1}{(\sum_{j=1}^{i-1}\frac{1}{j})^2}(S+\theta_W^2\sum_{j=1}^{i-1}\frac{1}{j^2})$
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$\xi_i(x)$ can be $0$ or $1$ obviously, $1$ if there is a mutation with size $i$ at position $x$.
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